Streams the Trail Log's current filter set as a CSV download —
GET <dashboard_path>/activity/export.csv.
Accepts the same query params the Trail Log LiveView patches into its own
URL (status, q, or range as one of 24h/7d/30d/all), so
"Export CSV" always downloads exactly what's on screen — see
SquatchMail.Web.Live.TrailLog.filters_from_params/1, the single source
of truth both this controller and the LiveView build Tracker filters
from. Rows are streamed with Plug.Conn.chunk/2 rather than built as one
giant binary, so a large export doesn't hold the whole CSV in memory at
once.
This is a plain Plug (same pattern as SquatchMail.Web.AssetController
and SquatchMail.Web.WebhookController) — a CSV download needs no
view/format negotiation.